1. Field of the Invention
The present invention relates to cross-country skis utilized for moving on snow using racing techniques on cross-country tracks, while going up, going down and on uneven terrain.
2. Description of Background and Relevant Information
Cross-country skis generally comprise a central portion which is slightly bent between a front contact line and a rear contact line, with a front end zone curved upwardly to form a spatula in front of the front contact line, and with a rear end zone generally slightly curved upwardly, at the rear of the rear contact line.
When the cross-country ski is positional on a plane, through its lower slide surface, at rest, it rests on the plane through two transverse lines, referred to as front and rear contact lines. Between the two front and rear contact lines, the slightly bent central portion detaches from the support plane, from the front contact line, the distance increases progressively between the support plane and the lower slide surface of the ski, until reaching a maximum in the central zone of the ski, then this distance diminishes progressively until reaching the rear contact line.
For many years, the traditional racing technique for cross-country skiing, for progression on flat ground and when going uphill, has comprised alternating steps. According to this technique, the two skis remain parallel in the direction of movement, the skier alternatively pushing on one ski to make the other slide. For the support to be efficacious, so as to give a good impulsion towards the front, one has always sought to give to the ski a particular structure in which the slight bend of the central zone of the ski has a relatively pronounced elbow in its center: from the front line of contact, the ski resting on a flat surface, the traditional slight bend is such that the distance increases progressively along a slope which diminishes slightly until reaching the central zone of the ski. In the central zone of the ski, the slope decreases very rapidly, to reverse, and then diminishes slightly when one moves in the direction of the rear contact line, until reaching this rear contact line. In the central zone of traditional skis, the modulus of variation of slope is relatively constant and is on the order of about 10.sup.-3 to about 2.10.sup.-3 cm.sup.-1. By virtue of this strong variation in slope, such a traditional slight bend constitutes, adjacent to the central zone of the ski, a portion which is slightly lifted from the support plane when the skier is simply resting on his skis. To apply this central zone against the support plane, it is necessary to give to the ski an impulse from top to bottom with a relatively substantially force.
Such a slight bend configuration, having a pronounced elbow, makes it possible, in the traditional technique of alternating steps, to reserve a central zone of the ski having good qualities of adherence on the snow. Thus, in the absence of impulse from top to bottom on the ski, the central adhering zone can rest slightly lifted from the snow, and not oppose sliding of the ski. On the other hand, by giving an impulse from top to bottom, the skier can apply the central adhering zone on the snow, thus taking advantage of this adherence for support and to progress forwardly.
To obtain good sliding qualities despite the presence of the central elbow, it has been necessary, in conventional cross-country skis, to make the ends of the ski particularly flexible. One can characterize this flexibility by subjecting the ends of the ski to flexions under weight. Thus, when a length of 570 mm. of ski of a traditional cross-country ski, beginning at the front line of contact, is retained at its ends by free support and is subjected at its center to a substantially perpendicular force of 200 newtons, the elastic displacement of its central zone is about 200-350 mm.
Likewise, if a similar length of ski, adjacent the rear contact line is subjected to a similar test, the displacement of the central zone is approximately 100-250 mm.
For these flexibility tests, the contact lines are determined in a particular manner as will be seen below, so as to obtain a good reproduceability of the measures.
More recently, the technique of cross-country skiing has evolved, and it is now oriented towards movement by means of a skating step. In this technique, the skier places his skis along slightly diverging directions with respect to the direction of movement, and alternatively pushes on one ski and the other: at each step, one of the skis slides substantially flat while the other serves as a support and is for this reason inclined to make the interior edge of the ski bite.
However, it appears that the conventional cross-country skis are not adapted for such a skating technique.
In particular, the great flexibility of the front and rear ends of a traditional ski do not make it possible to take advantage of a good support on the interior edge to give a skating step impulse. And if one increases the rigidity of the ski, one obtains a cross-country ski whose sliding qualities are quite insufficient and are no longer compatible with the anticipated utilization.